This invention relates to a heat exchanger installed in a fluidized bed.
To effect heat exchange between a hot (or cold) fluid and another fluid, a fluidized bed type heat exchanger is generally used in which one or more heat transfer pipes are embedded in a fluidized bed consisting of numerous particles accumulated on a perforated distributing plate at the bottom of the fluidized bed. Thus heat is transferred to the second fluid through the particles and the walls of the heat transfer pipes. Usually, the particles are made of any heat resistant material such as silica, alumina and glass having a particle size of 0.1 to 2 or 3 mm. To ensure high heat transfer between the hot fluid and the heat transfer pipe in a limited space, it is necessary to increase the contact area between the fluid and the heat transfer pipe. To this end it has been the practice to provide fins for the heat transfer pipes.
FIG. 1 shows one example of a prior art heat exchanger installed in a fluidized bed, which comprises a vertical cylindrical casing 3 having a circular or rectangular cross-section and inlet and exit ports 1 and 2 at both ends for a hot fluid, in this example hot gas, a perforated distribution plate 4 at the bottom and a fluidized bed 6 constituted by numerous fluidizable particles 5 are formed on the distribution plate 4, and a serpentine heat transfer pipe 8 embedded in the fluidized bed 6 and provided with a plurality of fins 7. Although in this example turns of serpentine pipe 8 are parallel with each other, this arrangement is not material to this invention. The fins usually made of thin sheets of heat conductive material, copper or aluminum, and have a thickness of about 1 mm or less.
Fluid to be heated is admitted into the heat transfer pipe 8 through an inlet port 9, heated by the hot gas and then issues through an outlet port 10.
The hot gas passing through the perforations of the distribution plate 4 rises upwardly through the interstices between the particles in spaces between adjacent fins 7 in the form of small bubbles. Such bubbles combine into larger bubbles B (see FIG. 2) which prevent intimate contact between the particles 5 and the fins 7 and the pipe 8, thus decreasing the quantity of heat transferred therebetween. As the size of the gas bubble increases it prevents contact between the particles and the fins as shown in the left upper portion of FIG. 2. Although this phenomenon does not occur uniformly throughout the fluidized bed, once it occurs it decreases the flow resistance against gas flow so that the gas can readily flow through the spaces between the fins containing such large gas bubbles, whereby the gas flow becomes nonuniform thus decreasing heat transfer. In other words, the fins provided for the purpose of increasing the heat transfer area cause an adverse effect.
In a certain case, a heat exchanger installed in a fluidized bed is not provided with fins, and turns of the serpentine heat transfer pipe extend in the horizontal direction and are aligned in the vertical direction. With such construction, the particles are moved along both sides of the pipe to transfer heat. Accordingly, in order to increase the capacity of the heat exchanger it is necessary to increase the number of the horizontal turns of the heat transfer pipe, and hence the height thereof, thus increasing the flow resistance to the hot gas (or liquid). On the other hand, when the width of the heat exchanger is increased, uniform flow of the hot gas becomes difficult, meaning non-uniform flow of the particles as well as lowering of heat transfer efficiency. Furthermore, the particles are moved along both sides of the heat transfer tube but not in the longitudinal direction thereof.
At the upper surface of the fluidized bed, since the flow resistance of the particles to the flow of hot gas dissappears, the gas bubbles forcefully rupture to cause the particles on the upper surface of the bed to fly upwardly. When the hot fluid comprises liquid, it entrains the particles. In any case, a portion of the particles is carried to the outside of the heat exchanger requiring frequent replenishment of the particles.